For example, a coder or a potentiometer has a shaft that is rotationally mobile about an axis in relation to the body. The sensors make it possible to define the angular position of the shaft about its axis of rotation in relation to the body. An operator turns the shaft about its axis by means of a button secured to the shaft. The rotation of the shaft makes it possible for the operator to select a datum from a range. The range may be continuous and the selection is for example made by means of an analog potentiometer. The range may comprise a series of discrete values and the selection may be made by means of a coder, the rotation of the shaft making it possible to increment the values according to the pitch of the coder.
Other data input devices, such as a track ball, have a part that is mobile according to two rotational degrees of freedom. The mobile part is a sphere. The sensors deliver information on the position of the sphere according to the two degrees of freedom. The track ball can be used to point to an object on the surface of a screen. Each of the two rotations of the sphere is associated with a translation on the surface of the screen. In other words, a track ball can be used to point to objects on a screen of a computer system. An operator moves the sphere with the fingers to reach a desired position on the screen and thus point to an object displayed on the screen.
The invention is of particular use for data input devices belonging to equipment mounted onboard an aircraft. The data input device can then be used to select flight parameters displayed on the screen of the system, the selection being able to be made in the form of a choice of a value from a series by means of a coder or of a potentiometer or by pointing to an object displayed on a screen.
The aircraft may be subject to turbulences which risk disrupting pointing to the objects. More specifically, the turbulences generate vibrations that can result in uncontrolled movements of the mobile part. Even in the absence of established turbulences, slight acceleration phenomena affecting the equipment prevent the crew members from pointing to a graphic zone with any more than a certain level of precision.
For the track ball, a usual solution to this problem consists in overdimensioning on the screen the objects that can be picked so as to take account of this limitation. Consequently, this reduces the number of objects that can be displayed on a given display surface area.
Attempts have also been made to stabilize the hand of the operator by creating suitable bearing surfaces. An example is given in the patent application published under the number EP 1 552 376 in which the fixed part of a track ball has an ergonomic form intended to receive the palm of the operator, hence its name of “palm rest”. Nevertheless, such a palm rest does not make it possible to completely stabilize the sphere through the fingers of the operator.
These two solutions are often associated with sphere braking means. These means make it possible to limit the uncontrolled movements of the sphere in the case of vibrations.
The brake is naturally secured to the fixed part of the track ball and exerts a load on the sphere for example by means of a skid rubbing against the sphere. In the displacements of the sphere, the bearing of the skid generates a friction torque opposing the rotations of the sphere. The skid is kept pressed against the sphere by means of a spring bearing against the fixed part.
Assuming that the sphere is accessible to the fingers of the operator from above, one solution already implemented consists in positioning a surface rubbing on the ball above the plane of symmetry of the sphere. The rubbing surface is generally produced by a membrane made of plastic material. The force exerted along the vertical axis of the sphere by the rubbing surface is produced by an elastomer or a spring system pressing on the rubbing surface.
This arrangement causes many problems. The surface area that can be used by the operator is reduced because of the presence of the rubbing surface above the plane of symmetry of the sphere.
The friction torque generated by the brake is greatly dependent on the accuracy of coaxial alignment between the vertical axis of the sphere and the axis of the friction surface. The sphere is generally placed on pivots forming sensors that make it possible to deliver information on the position of the sphere in relation to the fixed part. The separation between the axes of the sphere and of the brake depends on a chain of dimensions involving numerous mechanical parts. The accuracy of coaxial alignment requires precise assemblies and machinings.
The friction torque generated depends on the pressure exerted by the elastic device which in turn depends on the vertical position of the sphere. To limit this dependency, it may be necessary to provide means for adjusting the brake vertically. This solution is costly because the adjustment means have to be set individually for each track ball.
The torque generated also varies greatly as a function of the variations of the diameter of the sphere, particularly upon temperature variations, causing it to rise against the braking device when being used at high temperature or to drop when being used at low temperature.
It has been found that the brake is particularly fragile to use outside of its normal position of use such as when turned over during transportation.
Moreover, the friction torque is set when the track ball is mounted. To modify this setting, it is necessary to dismantle the brake, which is difficult to accept in the aeronautical field.
Conventional brakes exhibit dry frictions. In other words, a non-zero break-away torque is necessary to set the mobile part of the track ball in motion. The break-away torque has to be as weak as possible to allow for accurate pointing on the screen. It must not be too weak for the ball to be set in motion alone and move the cursor on the screen in use in a vibratory environment.
Furthermore, when the mobile part is in motion, after it has broken away, the friction torque must not be too high to remain comfortable for the operator. The range of torques acceptable to an operator is relatively low.
Many conditions of use cause the friction torque to change such as, for example, fat from the fingers of the operator or dust deposited on the sphere, differential expansions as a function of the temperature variations of the mechanical parts that make up the track ball, the relaxation of the braking device, the wear of the braking device, etc.
The friction and break-away torques are greatly variable. Their control requires a perfect knowledge of the impacts of each factor of degradation of these torques to best position the torque value at the time of the delivery of the track ball to take account of the variations over time.
Finally, each operator has a perception of these torques and their operational impact. Some operators prefer a higher torque for a fine adjustment, others prefer a weak torque for rough but rapid adjustments to the cost of fine adjustment. The braking torque cannot be personalized in operation by the operator without dismantling the braking device and readjusting.
In the current solutions, all these constraints are taken into account to deliver a track ball set with precise break-away and friction torques that lie within a narrow tolerance. This adjustment is made by successive iterations before the delivery of the equipment. It is repeated regularly in operation when the operator feels that the torque is no longer satisfactory for suitable use.